Rheumatoid arthritis (RA) is mediated in large part by two major effector cells; effector memory T (TEM) lymphocytes and fibroblast-like synoviocytes (FLS). During RA, these cells have phenotypes that result in joint destruction through a variety of mechanisms. TEM cells induce inflammation throughout the synovial joints mediated by the release of a variety of pro-inflammatory cytokines. FLS are resident joint cells that normally function to maintain synovial homeostasis. During RA, their phenotype becomes altered in that they become highly invasive, lose contact inhibition, induce angiogenesis, function as professional antigen-presenting cells, and release pro-inflammatory cytokines. Both of these phenotypes in TEM cells and FLS result in inflammation and joint degradation. It is also likely that these cells interact with each other during RA, either directly through the FLS ability to act as antigen-presenting cells or indirectly through cytokine-induced signaling. The K+ channels Kv1.3 on TEM cells and KCa1.1 on FLS regulate each cell's pathogenic phenotypes. Therefore, I hypothesize that Kv1.3 and KCa1.1 can serve as regulators of the interactions between TEM cells and FLS and a combined therapy of Kv1.3 and KCa1.1 blockers can dramatically reduce disease severity in an in vivo RA model through a synergistic effect. The overall goal of this project is to determine if the interactions of FLS and TEM cells that results in their activation cn be hindered by modulating Kv1.3 and KCa1.1. This will be beneficial as it will provide evidence regarding the benefits of combined therapies targeting both TEM cells and FLS for treating RA. The first objective of this project is to determine if a combined therapy of KCa1.1 and Kv1.3 blockers is more efficacious than single blocker treatments at ameliorating disease severity in a rat model of RA. Rats with a model of RA will be treated with either a single channel blocker, both blockers, and vehicle. Clinical signs of disease will be monitored in each group and histology and x-rays will be used to assess joint damage. Then, ex vivo functional analyses of T cells and FLS isolated from animals in each treatment group will be completed. The second objective is to determine if the cyclical interactions and activations of TEM cells and FLS are mediated by Kv1.3 and KCa1.1 in vitro. This will involve examining the phenotype of FLS and TEM cells co- cultured in the presence or absence of KCa1.1 or Kv1.3 blockers, or from cells with either channel knocked down by siRNA. Proliferation and cytokine production will be examined in both cell-types. FLS expression of molecules needed to directly interact with a T cell will also be examined. Furthermore, it will be determined if FLS can induce differentiation of central memory T cells into TEM cells and if this is dependent on KCa1.1 activity. Overall, this work will validate KCa1.1 and Kv1.3 as regulators of the interactions between FLS and TEM cells and define the benefits of a combined therapy for treating a rat model of RA.